This invention relates generally to identification systems and in particular to identification of individuals by comparison of fingerprint patterns.
It has long been recognized that certain personal features are unique to an individual and can be used as a means of positive identification. One such feature that is frequently chosen for this purpose is the fingerprint. U.S. Pat. Nos. 3,383,657; 3,527,535; 4,185,270 and 4,210,899 all describe identification systems which are based on fingerprint pattern recognition techniques.
In general, techniques such as those described in the above-mentioned patents involve scanning a selected fingerprint pattern, converting the pattern information into an electrical signal, and storing or processing the information (e.g., by comparing it to prestored information) to effect an identification. Scanning is generally accomplished by opto-electronic means which avoids the use of inks or other undesirable materials. U.S. Pat. No. 3,527,535 to Monroe describes several methods using different types of glass prisms and illumination means to produce a visible or scannable fingerprint pattern. Other systems described in the literature have used an illuminated right angle prism and the technique of frustration of total internal reflection to produce a visible and scannable image. These systems depend for their success on the fact that the ridges of the skin on the finger contact a smooth, clean optical surface while the valleys do not. While it is seldom mentioned in the literature, the practical application of such an optical system requires a thorough cleaning after each use to remove the oils, water and dirt deposited on the surface by the finger. If these deposits are not removed, a residual image caused by them may be seen and subsequent operation of the system will be severely affected.
Another problem often experienced in fingerprint recognition systems involves registration of a fingertip in the proper position for scanning. Some existing systems use a jig or guide in an attempt to position the finger in the same location and orientation on the scanner for each use. While this is generally a desirable feature, it cannot be relied on to produce re-registration of the fingerprint to the exact same location on the surface of the optical element of the scanner for every subsequent scan performed. Accordingly, some degree of misregistration usually occurs. In an image matching system, there will be some degree of correlation between similar, but not identical, images. Thus, a system which does not provide accurate registration capability will experience difficulty in distinguishing between similar images and misregistered identical images.
Difficulties in identifying a known fingerprint pattern by comparison to a subsequently scanned identical pattern can also be caused by distortion. As used here, the term distortion refers to changes in the relative locations of ridges and valleys in a fingerprint pattern which can result from a number of causes. For example, the flesh beneath the skin is plastic and can be deformed with reasonable amounts of force or pressure. This property allows a relatively large area of a fingerprint pattern to be scanned since the fingertip is generally pressed against a flat optical element surface for scanning. However, a certain degree of relative distortion between fingerprint patterns scanned at different times can result when different amounts of pressure are applied in pressing the fingertip against the optical surface. Non-uniform pressure or force components acting on the fingertip which are not normal to the flat optical surface can also result in distortion being introduced. Considering the plastic nature of the flesh and skin surrounding the bone on the finger, it is easy to see that such forces can cause pattern displacements equal to several ridge-valley separations. Other factors, such as large weight gains or losses by the individual and large ambient temperature variations, have also been shown to produce relative distortions in fingerprint images. While the effects here are relatively small (generally on the order of less than 3 percent), they can be sufficient to adversely affect the accuracy of a comparison.
An object of the present invention is to provide an apparatus which prevents deposition of dirt, oils and water on the surface of an optical element in a scanning device.
Another object of the present invention is to provide a method and apparatus for comparing data arrays representative of fingerprint patterns which compensates for misregistration of the fingertip on the scanner.
A further object of this invention is to provide a method and apparatus for comparing data arrays representative of fingerprint patterns which compensates for distortion, relative to an original scan pattern, introduced into fingerprint patterns produced by subsequent scans.
These and other objects are attained in an apparatus for identifying fingerprint patterns comprising: an optoelectronic scanner for scanning fingerprint patterns; threshold circuits for reducing the electronic signal to a binary array of 1's and 0's; memory for storing the initial or reference data arrays; additional memory for storing subsequent or new scanned data arrays; means for comparing the reference and new data arrays; means for determining the degree of correlation between the reference and new data arrays; means for determining the degree of distortion between the reference and new data arrays; and means for producing an indication signal when the correlation value exceeds a pre-set threshold and when the distortion value is below a pre-set threshold.
The reference data array will be referred to as an X.times.Y array and the new scan data array will be referred to as an M.times.N array. In the example to follow M&gt;X and N&gt;Y.
A preferred method of comparing the M.times.N data array to the previously stored X.times.Y reference data array comprises the steps of: dividing the X.times.Y array into a plurality of sub-arrays; comparing individual elements of an X.times.Y sub-array with corresponding elements of a sub-array of the M.times.N array and generating a correlation value indicative of the degree of correlation existing between the corresponding elements; repeating the comparison step for a number of sub-arrays of elements of the M.times.N array; identifying a highest correlation value generated in these comparisons; repeating the comparison steps for each of the plurality of X.times.Y sub-arrays; and comparing the highest correlation values identified to a predetermined threshold value to determine whether or not a sufficient correspondence exists to indicate a match. As will be described in more detail below, the X.times.Y reference array in one embodiment of the invention is preferably divided into four sub-arrays (i.e., quadrants). Comparison of the elements of each sub-array with a number of sub-arrays of elements of the M .times.N array results in compensation for both misregistration and distortion introduced when a scan is performed to generate the M.times.N array.
The comparison of elements of the X.times.Y sub-arrays and the M.times.N sub-arrays is preferably performed by executing a logical "exclusive or" operation, using the elements of the X.times.Y sub-array and the elements of the M.times.N sub-array as inputs, to generate a correlation value by counting the number of matches which occur when all elements of an X.times.Y sub-array are compared to all corresponding elements of an M.times.N sub-array. In an especially preferred method, the location in the M.times.N array of the sub-array having the highest degree of correlation with the X.times.Y sub-array is identified. The displacement of the sub-arrays having the highest correlation values from the location which would result from scanning a perfectly registered and undistorted fingerprint pattern provides a measure of the misregistration and relative distortion of the pattern being compared. This distortion measure can be used, along with the peak correlation values themselves, to determine if a match has occurred. Thus, an accurate comparison can be made even in the presence of misregistration and distortion. An especially preferred method involves summing the highest correlation values generated for each X.times.Y sub-array and corresponding M.times.N sub-array, dividing the total by the measure of relative distortion to produce a correlation value-to-distortion ratio, and comparing the ratio to a pre-determined threshold value.
A preferred method of processing the correlation values as they are generated by the comparisons described above involves creation of a correlation array whose elements represent the degree of correlation existing between elements of an X.times.Y sub-array and the elements of each of the M.times.N sub-arrays to which the X.times.Y sub-array is compared.
An advantageous embodiment of the present invention includes additional apparatus to position the fingertip over a viewing surface of an optical element of the scanning device. An especially advantageous embodiment includes provision of a transparent film over the viewing surface to prevent the fingertip from contacting the surface directly. The transparent film can be moved, or indexed, relative to the viewing surface, to provide a clean contact surface for the fingertip on subsequent scans. Indexing of the film preferably takes place automatically after a comparison is performed. The transparent film apparatus may include a length of clear, plastic film, a supply reel, a take-up reel and drive means for at least one of the two reels.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.